Intelligent structures absorb vibration and noise

A tuning fork is an example of decreasing vibration. Image: Fraunhofer LBF

Vibration and noise are closely connected. Both are ubiquitous in our daily lives, which are filled with disruptive sounds: road noise, air conditioners humming in office buildings, machines rumbling in industrial production. Vibration is also omnipresent, disturbing us and affecting product quality in cars, on trains, aboard aircraft or in modern production.

At a joint research centre in Darmstadt, researchers at TU Darmstadt and the Fraunhofer Institute for Structural Durability and System Reliability LBF are developing new intelligent material and structure systems that neutralise unpleasant or harmful vibration and noises. Their goal is to build structures that will automatically adjust to any disturbances. Such adaptable systems – termed 'adaptronic' by researchers – are necessary because vibration and noise, or more precisely, the excitation or reaction of the structures, are in constant flux, for example in an accelerating car.

Sensitive sensors

Generally speaking, systems are adaptronic when they are able to self-adapt to altered external conditions. With a minimum of energy and little material input, adaptronic systems adapt to operations and reduce environmental disturbance. In order to achieve this, new, intelligent materials are integrated into the structure. A passive structure thus becomes an active one, constantly re-adjusting and adapting optimally to the current situation. The principle is simple: sensors are embedded into the material and measure the sounds or frequencies that constitute a noise. The sensors register the vibrations and the behaviour of the structure.

Also embedded into the material are deformable components, called actors, which convert the sensors' measuring signal into counter-vibrations. Vibration and counter-vibration interfere with and almost neutralise one another, which is called destructive interference, and the noise becomes much quieter. As early as 1500, Leonardo da Vinci formulated this concept for active noise and vibration compensation as a thought experiment. Only today are researchers able to translate this idea into commercially viable technical products.

Less maintenance, more safety

Adaptronics are highly relevant when it comes to developing and implementing innovative and efficient lightweight structures, in particular in vibroacoustics where vibration and noise are coupled together. In the future, car bodywork, open-plan office dividers, glass facades or vibration absorbers in vehicles could be made from adaptronic materials. Such materials could greatly advance lightweight construction, since lightweight materials like carbon fibre composites vibrate more easily and thus absorb less noise than heavier metals. Adaptable systems to control noise and vibration could easily compensate for this disadvantage. The benefits for modern products are obvious.

Embedded sensors can also help reduce the amount of maintenance required for light aircraft or out-of-the-way wind turbines at sea. They can constantly monitor the condition of carbon fibre components, detect even minuscule defects invisible to the human eye and signal them back to a computer. Maintenance then only has to be conducted when necessary, a gain in safety despite a reduction in maintenance work.

Darmstadt researchers have already scored successes with systems that work in model applications. Now they want to build adaptronic systems with the help of newly developed materials such as ceramics and plastics that can be used more widely and diversely.

Adaptronic materials are considered the key to a low-noise, safe world of lightweight construction materials. And this key might just come from Darmstadt.